The technique of lithography is based on immiscibility of oil and water. Oil materials or ink is preferentially retained in image regions, and aqueous solutions are selectively retained in non-image regions. When surfaces of plate materials suitably prepared are wetted with water, followed by coating with print ink, the non-image regions hold water to repel the ink, whereas the image regions receive the ink to repel water. Accordingly, when these plate materials are brought into contact with surfaces to be printed, directly or indirectly through intermediates called blankets, the ink on the image regions is transferred to perform printing.
As materials for forming the ink-receiving image regions, many organic materials are known. They are basically formed from light-sensitive components (radiant ray-sensitive materials) and binders. As the radiant ray-sensitive materials, many materials are known. Useful negative type compositions include diazo resins, photo-crosslinkable polymers and photo-polymerizable compositions. Useful positive type compositions include aromatic diazo-oxide compounds such as benzoquinonediazides and naphthoquinonediazides. When imagewise exposure is given to these materials, followed by development and optional fixing, image regions of imagewise distribution are formed which can be used in printing.
As a material for forming the water holding non-image regions, an anodized aluminum surface has generally been used. For preparing aluminum for this application, both the graining process and the subsequent anodization are generally performed. The graining process is useful to improve the adhesion of the radiant ray-sensitive paint films, and also useful to enhance the water holding characteristics of the non-image regions of lithographic printing plates.
Such hydrophilized surfaces are exposed at non-image areas by exposure and development, and when fountain solutions are given thereto, they are sufficiently retained. Accordingly, the print ink is effectively repelled to inhibit stains in printing.
The above-mentioned ordinary lithographic printing plates are required to be developed with developing solutions after imagewise exposure. The developing solutions remove the non-image regions of image forming layers to expose the surfaces of supports hydrophilized by roughening thereof. The developing solutions are typical aqueous alkaline solutions, and sometimes contain organic solvents in large amounts. The development therefore requires not only its complicated processing procedure, but also waste disposal of large amounts of the aqueous alkaline solutions. Accordingly, this has been an important concern in the printing field for a long period of time. In recent years, the problem of the alkaline developing waste liquid has been noted particularly from the standpoint of environmental preservation, and methods for reducing the amount of waste liquid as small as possible and measures for lowering alkalinity have been proposed. However, no fundamental solution has been found.
From the above-mentioned background, efforts to produce printing plates requiring no development using alkaline developing solutions have been made. In recent years, for example, methods for preparing printing plates by use of laser exposure have been known. However, printing materials used herein mostly form images by ablation.
On the other hand, JP-A-9-169098 (the term "JP-A" as used herein means an "unexamined published Japanese patent application) proposes a method of using a ZrO.sub.2 ceramic material as a surface material, and changing the surface properties by laser irradiation to form an image. In this system, the ceramic material itself has sensitivity to laser beams, and corresponds to image formation (ink-receptive) and deletion (hydrophilization) at different wavelengths. Accordingly, both the image formation and deletion can be carried out only by irradiation of laser beams.
However, for the lithographic printing plates requiring no development processing which have hitherto been proposed, sufficiently satisfactory practicability has not been obtained. That is to say, in a field to which a printing system is applied, extreme accuracy is generally required for maintaining the quality of images. Accordingly, the use of printing materials which form images by ablation undesirably causes mist produced by the ablation to become a source of pollution in the system.
Further, the printing material using the ZrO.sub.2 ceramic material described in JP-A-9-169098 is very low in the degree of changes in polarity. Hence, when the surface is contaminated by some chance, there is the high possibility that ink adheres to a non-image area in printing practice to form a stain.